Contact lenses can be manufactured economically in a mass production manner by a conventional cast-molding process involving disposable molds (e.g., PCT published patent application No. WO/87/04390, EP-A 0 367 513, U.S. Pat. No. 5,894,002, all of which are herein incorporated by reference in their entireties) or by an improved cast-molding process involving reusable molds and curing under a spatial limitation of actinic radiation (U.S. Pat. Nos. 5,508,317, 5,583,163, 5,789,464 and 5,849,810). A critical step in the production of lenses using molds is mold opening and lens releasing from the mold without damaging the lens. Subsequent to the completion of the contact lens molding process, the polymerized lens tends to strongly adhere to the mold. During mold opening and removing the contact lenses from the mold, cracks, flaws and/or tears may occur in the lenses or in the worst case the contact lenses even break totally. Contact lenses having such defects have to be discarded and lower the overall production yield.
Several methods have been developed or proposed. One method for releasing lenses is to hydrate the lens, namely, a lens-in-mold assembly after mold separation is placed in a hydration tank filled with water. Often hydration alone does not release the lenses from the molds. The lenses must then be gently removed from molds by hand. Such hand-assisted lens removal increases the likelihood of lens damage. U.S. Pat. No. 5,264,161 discloses an improved method for releasing a lens from a mold, in which surfactants are added to the hydration bath to facilitate the release of lenses from molds. However, the utilization of surfactants in a hydration bath does not provide a more effortless mold separation. Lens damage incurred during mold separation may not be minimized by hydrating lenses.
Another method of lens release is to incorporate surfactants as internal mold releasing agents into molds themselves as illustrated by U.S. Pat. No. 4,159,292. Incorporation of internal mold releasing agents in molds can decrease adhesion between lenses and molds. However, when a mold is used repeatedly, surfactants as internal mold releasing agent can be exhausted by exudation.
A further method of lens release is to apply external mold releasing agents (e.g., surfactants) in the form of a film or coating onto to the molding surfaces of a mold (e.g., those disclosed in U.S. Pat. Nos. 4,929,707 and 5,542,978). When external mold releasing agents are used, a portion of the agents used for treating the molding surfaces of the mold can migrate to the surface and interior of the polymerized lens.
A still further method of lens release is to incorporate internal mold releasing agents into a lens-forming composition for making contact lenses. The internal mold releasing agent can be a surfactant (U.S. Pat. Nos. 4,534,916, 4,929,707, 4,946,923, 5,013,496, 5,021,503, 5,126,388, 5,594,088, 5,753,730) or a non-polymerizable polymer (U.S. Pat. No. 6,849,210). By incorporation of an internal mold releasing agent in a lens-forming composition (or lens formulation), the adhesion between molds and lenses may be reduced, a relatively smaller force may be required to separate mold, and lenses may be removed from molds with less effort. A portion of the internal mold releasing agent need migrate to the surface of the polymerized lens in order to be effective to reduce the adhesion between molds and lenses. A great effort has been made to develop technologies for cast molding of hydrogel contact lenses with high precision, fidelity and reproducibility and at low cost. One of such manufacturing technologies is the so-called LIGHTSTREAM TECHNOLOGY™ (CIBA Vision) involving a lens-forming composition being substantially free of monomers and comprising a substantially purified prepolymer with ethylenically-unsaturated groups, reusable molds, and curing under a spatial limitation of actinic radiation (e.g., UV), as described in U.S. Pat. Nos. 5,508,317, 5,583,463, 5,789,464, and 5,849,810.
However, there are some practical limitations which hinder realization of all of the great potentials of such technology in the production of silicone hydrogel contact lenses. For example, when a silicone-containing prepolymer disclosed in commonly-owned U.S. Pat. Nos. 7,091,283, 7,268,189 and 7,238,750 is used to prepare a silicone hydrogel lens formulation, an organic solvent is generally required to solubilize the prepolymer. When such lens formulation is used to produce silicone hydrogel according to the Lightstream Technology™, the cured lens in the mold after UV crosslinking is still swollen in the organic solvent before the solvent exchange to water. Such lens may not be able to survive the mold opening and de-molding process, because the cured lens is in the swollen state in the organic solvent and has an inadequate stiffness and toughness (e.g., too low). As such, the production yield may be low and the production cost could be higher due to low production yield derived from the lens defects created during mold opening and de-molding process. However, conventional release mold agents are not effective to reduce lens defects created during mold opening and de-molding process in manufacturing contact lenses from silicone-containing prepolymers. The defects created during mold separation cab be a big issue in manufacturing contact lenses with silicone-containing prepolymer according to the LIGHTSTREAM TECHNOLOGY™.
Therefore, there is a need for a method for using a new mold releasing agent for molding contact lenses. There is also a need for a method for using a new mold releasing agent for molding silicone hydrogel contact lenses. There is a further need for a process for cast-molding contact lenses with an enhanced quality and enhanced yield achieved by reducing mold separation force and lens-mold adhesion through using a new mold releasing agent for molding silicone-containing prepolymer contact lenses with LIGHTSTREAM TECHNOLOGY™.